The simpler explanation, with a grounding in marketing and cognitive science: we basically train ourselves to ignore ads. If successful our active brain doesn't

The simpler explanation, with a grounding in marketing and cognitive science: we basically train ourselves to ignore ads. If successful our active brain doesn't register, while our passive brain might. Have we recently activated subject X in conversation, we notice ads around X a bit more. Hence the creepy feeling. Another explanation might be: I just discussed a holiday and promptly got an ad for the exact island. The person visiting me probably uses Facebook, so FB knows we were together and knows where he went via location. Logical conclusion: people discuss intercontinental trips and hence: show ads.